This invention relates to an electric power steering apparatus with an electric motor as a source generating a steering-assist force.
Steering of automotive vehicles is realized by transmitting a rotary operation of a steering wheel disposed in the vehicle's cabin to a steering mechanism provided outside the cabin for adjusting directions of the wheels (generally front wheels).
The electric power steering apparatus for use in an automotive vehicle, as shown in FIG. 1, includes: a first steering shaft 102 connected to a steering wheel 101; a torque sensor 105 sensing a steering torque by a relative rotary displacement of a second steering shaft 104 to the first steering shaft 102, both being linked via a torsion bar 103; and a reduction mechanism 109 having a worm 107 and a worm wheel 108, reducing a rotary motion of an electric motor 106 for steering assistance, which is driven based on a sense result of the torque sensor 105, to transmit the reduced rotary motion to the second steering shaft 104, wherein operations of the steering mechanism responsive to the rotary motion of the steering wheel 101 are assisted by a rotary motion of the electric motor 106 so as to alleviate a physical load imposed on a driver in steering.
A worm shaft on which the worm is mounted and the second steering shaft on which the worm wheel is mounted are each supported by bearings at opposed ends in the longitudinal direction and each blocked from displacement in the radial direction and the longitudinal direction.
In a case where a worm and a worm wheel are employed as described above, there arise backlash between opposed tooth flanks. Therefore, in the prior art, when a worm shaft and a worm wheel were assembled, it was performed within machining accuracies of parts such that no backlash is produced. However, with inevitable dimensional errors inherent in a worm, a worm shaft, bearings for supporting the shaft, a worm wheel, a second steering shaft for supporting the wheel and the like, backlash was produced after the assembly with a relatively high ratio.
Further, in a recent, advanced case applied with a high-power steering-assist force, wear of teeth of the worm and the worm wheel has been increased, thereby, problematically disabling avoidance of backlash. In this case, the worm shaft and the second steering shaft are blocked in displacement, disabling adjustment of backlash, and therefore unpleasant noise caused by backlash is leaked into the cabin of an automotive vehicle.